1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus, and more particularly to a sheet feeding device which feeds sheets while separating the sheets one by one and an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
In the related art, an image forming apparatus, such as a printer, a copying machine, or a facsimile is equipped with a sheet feeding device which sends a sheet from a sheet stacking portion using a pick-up roller and which separates the sheets one by one using a separation portion and then feeds the separated sheets in a case where multiple sheets are fed.
The separation portion provided in the sheet feeding device is categorized as a roller type which uses a feed roller and a retard roller capable of reverse rotation or a friction separation type which uses a separating member such as a separation roller or a separation pad.
For example, the roller-type sheet feeding device includes a feed roller which rotates in synchronization with a pick-up roller in the same direction and a retard roller which is pressed by the feed roller with a sheet conveying path therebetween. A predetermined torque in an opposite direction to the sheet feed direction is exerted on the retard roller through a torque limiter. Accordingly when a predetermined torque is applied to the retard roller due to the double feed of sheets, the retard roller rotates in the opposition direction to the sheet feed direction, so that the sheets double fed are separated one by one. Accordingly, in a case where, for example, a sponge roller is used for the retard roller, the collapse amount of the retard roller increases over use and the increased collapse amount results in a resisting force against the rotation, thereby impeding the retard roller's rotation (hereinafter, referred to as “co-rotation”) along with the feed roller. This gives rise to the co-rotation failure of the retard roller. The co-rotation failure of the retard roller impedes the feeding of the sheet into a nip portion between the feed roller and the retard roller, which is likely to cause a paper jam.
In particular, when the load resistance of the retard roller for separating sheets is heavy and the retard roller is a sponge roller, the problem with that retard roller is a significantly short durability life span compared with general conveying rollers. Furthermore, in recent years, with the increase in the amount of information, image forming apparatuses such as copying machines are in ever more increasing use. Accordingly, it is required to prolong the durability life span of the retard roller.
Meanwhile, since an increase in the collapse amount of the retard roller leads to a higher resisting force against the co-rotation of the retard roller, sheets which are double-fed can be separated. For such a reason, the rotary torque transferred to the retard roller can be reduced according to a durability status such as the number of conveyed sheets or the collapse amount of the sponge retard roller, the sheets may be properly separated and thus co-rotation failure can be suppressed. That is, it is believed that the durability life span of the retard roller can be prolonged by delaying the occurrence of the co-rotation failure as much as possible.
In this regard, there are more known sheet feeding devices. For example, Japanese Patent Laid-Open (JP-A) No. 2002-2992 discloses a sheet feeding device which reduces the rotary torque transferred to a retard roller using a plurality of torque limiters and JP-A No. 2003-301857 discloses a sheet feeding device which reduces the rotary torque by modifying the internal structure of a torque limiter.
However, those sheet feeding devices have the following problems. That is, in the case of using a plurality torque limiters as in the sheet feeding device disclosed in JP-A No. 2002-2992, the cost of the sheet feeding device increases. On the other hand, in the case of reducing the rotary torque with the modified internal structure of the torque limiter as in the sheet feeding device disclosed in JP-A No. 2003-301857, a highly precise and accurate calibration is required for the torque limiter.
Accordingly, the present invention provides a sheet feeding device capable of prolonging the durability life span of a retard roller using a simple structure with ease and an image forming apparatus which includes the sheet feeding device.